Ten-second staircase

It's a crime tailor-made for the Peculiar Crimes Unit: a controversial artist is murdered and displayed as part of her own outrageous installation. No suspects, no motive, no evidence--but this time they "do" have an eyewitness. A twelve-year-old claims the killer was a cape-clad highwayman atop a black stallion. Whoever the killer really is, he seems intent on killing off enough minor celebrities...